There are some really great photos/videos in this thread!
I've barely managed to muster the confidence to hit 400'; the main concerns I would have with flying over water would be birds (though if it's true that seagulls don't commonly attack them that's good news) and losing it to a flyaway or other failure, although it seems that this is far less likely with the P3s than it was with the previous generations. Perhaps I'll have to give it a go.